With the improvement of light-emitting efficiency, a light-emitting diode (LED) becomes adopted in a luminaire. Instead of an incandescent lamp using a filament as a light source, a bulb-type LED lamp using the LED as a light source becomes popular. The LED lamp includes a board on which the LED is mounted. In the LED lamp, since the LED as the light source is mounted on the flat board, a luminous intensity distribution angle of 180 degrees or more can not be obtained. Besides, since the light emitted by the LED has higher directionality than the light emitted by the filament of the incandescent lamp, the center of an irradiation field is bright and the periphery gives an impression of dimness.
In order to improve the luminous intensity distribution characteristics as stated above, an LED lamp is developed in which a board itself mounted with an LED is inclined to increase the amount of luminous intensity distribution to the side, or an LED lamp is developed which includes optical elements such as a prism and a lens or a reflecting plate.
In order to distribute light in a wide range, there is a case where plural boards mounted with LEDs are respectively arranged at different angles. The plural boards are required to be assembled three-dimensionally, and the respective boards must be cooled. When lighting-on and lighting-off are repeated, a temperature change occurs in the boards. When differences in the amount of heat expansion occur among the boards and cooling members thereof, there is a fear that the respective boards can not be uniformly cooled. For improvement of such problems, the structure becomes complicated and the manufacturing cost increases, and therefore, the improvement is hard to adopt.
Further, when the optical elements or the reflecting plate is attached in order to change the luminous intensity distribution characteristics, the efficiency of diffusing the light emitted by the LED must not be reduced. Besides, a shadow or unevenness of light due to the provision of these components must be prevented from occurring in a range of luminous intensity distribution. Accordingly, the range in which the optical element or the reflecting plate can be attached is limited. In the related art technique, the optical element or the reflecting plate is directly fastened to the board by a screw or is bonded by an adhesive. However, it is not preferable because the number of parts increases when the optical element or the reflecting plate is fastened to the board by the screw. Besides, when the optical element or the reflecting plate is bonded to the board by the adhesive, a shear force is applied to the bonding surface due to a difference between the coefficient of linear expansion of the board and the coefficient of linear expansion of the optical element or the reflecting plate. The service life of the LED as the light source is remarkably longer than the service life of the filament of the incandescent lamp, and as a result, the LED lamp is expected to be used for ten or more years. Thus, since the adhesive deteriorates over time, and the shear force is repeatedly applied, there is a fear that the adhesive surface is peeled or the adhesive part is damaged at the ending of the service life of the LED lamp.